Compound Bow
In modern archery, a compound bow is a bow that uses a levering system, usually of cables and pulleys, to bend the limbs. Overview The pulley/cam system grants the user a mechanical advantage, and so the limbs of a compound bow are much stiffer than those of a recurve bow or longbow. This rigidity makes the compound bow more energy-efficient than other bows, as less energy is dissipated in limb movement. The higher-rigidity, higher-technology construction also improves accuracy by reducing the bow's sensitivity to changes in temperature and humidity. The pulley/cam system also confers a benefit called "let-off." As the string is drawn back, the cams rotate. The cams are eccentric rather than round, and so their effective radius changes as they rotate. Each of a compound bow's two cams features two tracks: an inner track which connects to the opposite limb or opposite cam through cables, and an outer track through which the bowstring runs. As the bow is drawn, the ratio of bowstring pay-out and cable take-up relative to limb-weight and leverage of the cams changes. By manipulation of the shapes of these cam tracks, different draw-stroke profiles can be created. A compound bow can be soft-drawing with a slow build-up to peak weight and a gradual let-off with a long "valley" at the end. It can also be hard-drawing with a very fast build-up to peak draw-weight, a long plateau where weight is maintained, and a quick let-off with a short valley. The let-off itself is the result of the cam profiles having passed center and approaching a condition very similar to a cam-lock. In some compound bows, if the draw-stops or draw-length modules are removed, they will self-lock at full draw and require professional equipment to unlock safely. Many compound bows offer 70 - 85% let off once they pulled to full draw. This allows the shooter to relax and concentrate on the intended target at which they are shooting. A bow's central mount for other components such as the limbs, sights, stabilizers and quivers is called the riser. Risers are designed to be as rigid as possible. The central riser of a compound bow is usually made of aluminum, magnesium alloy, or carbon fiber and many are made of 7075 aluminum alloy. Limbs are made of fiberglass-based composite materials and are capable of taking high tensile and compressive forces. The limbs store all the energy of the bow – no energy is stored in the pulleys and cables. Draw weights of adult compound bows generally fall between 40 and 80 pounds, enabling arrow speeds of 250 to 370 feet per second. In the most common configuration, there is a cam or wheel at the end of each limb. The shape of the cam may vary somewhat between different bow designs. There are several different concepts of using the cams to store energy in the limbs, and these all fall under a category called bow eccentrics. The four most common types of bow eccentrics are Single Cam, Hybrid Cam, Dual Cam and Binary Cam. However, there are also other less common designs, like the Quad Cam and Hinged. Cams are often described using their "let-off" rating. As a cam is rotated, the force required to hold the bow in position reaches a peak and then decreases as the bow approaches maximum extension (a position known as "the wall"). The percent-difference between the maximum force encountered during the draw and the force required to hold the bow in full extension is the "let-off". This value is commonly between 65% and 80% of the peak weight for recently designed compound bows, although some older compound bows provided a let-off of only 50% and some recent designs achieve let-offs in excess of 90%. Compound bow strings and cables are normally made of high-modulus polyethylene and are designed to have great tensile strength and minimal stretchability, so that the bow transfers its energy to the arrow as efficiently and durably as possible. In earlier models of compound bows, the cables were often made of plastic-coated steel. History In the New World, the recurve bow or long bow were the main projectile weapons for the common infantryman in the Third Civilization, with the exception of the Parpaldia Empire, who had access to primative flintlock muskets. As a result, shields and armor were only able to protect against the slower-moving arrows, which struck with less force and had less range. Even the strongest bowman could only pull with enough force to reach a distance of under 200 yards. However, when the Riem Kingdom were able to get their hands on compound bows from Japan, the face of medieval warfare changed. Since bows were not covered under the Japan Technology Outflow Bill, the Japanese did not consider the huge advantages the bow provided due to the cam-pulley system, which were far ahead of the standard recurve and long bows. As a result, archers could now hit targets as far as 500 yards and the arrows were able to pierce the tough scales of land dragons. Category:Technology Category:Weapons